Text Box: Example: Even with the most onerous parameter which is for gas Group IIC 691m of Standard Digital LHDC may be installed. 0.083uF/120pF = 83000/120 = 691m If it is necessary to “interpose” a cable between the LHDC and the Barrier, then the parameters of this cable must also be considered to ensure the Installation Conditions are met. It is also necessary for such cable to have a screen in the case of Analogue LHDC. It is possible to employ other types / manufactures of barrier, however in addition to observing the intrinsically safe requirements of their certification, Patol does not recommend the use of barriers with less than 22V zener diode operation for Analogue LHDC. Digital LHDC Interfacing The interfacing of Digital LHDC requires consideration of the specification of the interface / fire panel trigger (monitoring) input together with the various factors of the LHDC circuit. A typical simplified schematic arrangement is shown in Figure 3.  Fire panel trigger circuits register a fire alarm when the trigger circuit current (I) increases beyond a set value. Most equipment has a further level which discriminates short circuit fault. An Alarm resistor (Ra) may need to be employed, with a value such that when the LHDC operates at either end (Fire Sw. - Fig.3) the circuit current is within the equipment ‘alarm window’. Example LHDC is 184 Ω/km (loop) : Say 300m installed, Rlhd = 0.184 x length(m) = 55Ω For both channels of the MTL7778 Barrier Rzb = 248Ω Say total hazardous area interposing cable resistance = 25R : non hazardous area = 10R Fire resistance - Min : Rih + Rzb + Ris = 25 + 1302 + 10 = 1337Ω - Max : 1337 + 55 = 1392Ω It would probably not be required to fit Ra in this case – The fire panel must respond to 1k4. Typically fire panel trigger circuits require an End Of Line resistor (EOL) in order to monitor the circuit for open circuit fault. It may be necessary to reduce the value of the actual resistor Reol fitted to accommodate for the series resistance of the rest of the circuit (Rlhdc+Rih+Rzb+Ris+Ra). Circuit total = 1392 : If panel requires 4k7 EOL then Reol = 4K7 – 1392Ω : Fit 3k3 say.

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Linear Heat Detectors - Installing LHDC using Intrinsically Safe Barriers - Page 3